1. Field of the Invention
The present invention relates to a regenerative braking method for a vehicle having an electric motor, and, more particularly, to a regenerative braking method for a vehicle having a regenerative braking function through the use of an electric motor, in which a hydraulic brake device can be controlled in consideration of a regenerative braking force.
2. Description of the Related Art
Conventional engine powered vehicles have several problems in that they require numerous devices, such as for example, an engine, transmission, drive shaft, hydraulic brake, fuel supply device associated with the engine, intake and exhaust devices, cooling and lubricating devices, vibration-proof device, etc. Also, the engine powered vehicles tend to cause air pollution due to exhaust gas. To solve these problems, currently, vehicles having an electric motor, hydrogen vehicles, other next generation vehicles using a fuel cell or solar energy, and the like are being developed.
Recently, in particular, vehicles having an electric motor have been actively developed. The electric motor vehicles employ a regenerative braking method suitable for the improvement of fuel efficiency.
In the vehicles having an electric motor, if the electric motor is actuated by electric energy, driving wheels of the vehicle are rotated by the electric motor. Accordingly, how efficiently electric energy is used to actuate the electric motor becomes the major problem to be solved in the electric vehicles. For this, it is desirable that the electric motor functions as a generator to generate electric energy and store the generated electric energy in a condenser if a speed reduction or braking command is given by a vehicle operator. By the way, while the electric motor functions as the generator, vehicle wheels are subjected to a braking force, which is called a regenerative braking force. Consequently, the vehicle wheels are subjected to the sum of the regenerative braking force by the electric motor and a hydraulic braking force/frictional braking force by a hydraulic brake device.
The generation of electric energy by the electric motor is possible even during operation of a brake. Specifically, in the vehicle having an electric motor, the electric motor is able to generate electric energy from heat energy emitted during operation of a brake. In this case, the amount of electric energy obtainable by the electric motor is variable in accordance with the speed of the vehicle, the charge amount of a battery, and the like.
Accordingly, to fulfill the vehicle operator's desire to brake, i.e. to achieve the amount of braking desired by a vehicle operator corresponding to the sum of the regenerative braking force and hydraulic braking force, there must be produced only the hydraulic braking force needed after subtracting the regenerative braking force generated by the electric motor from the vehicle operator's desired braking force.
However, this may cause the vehicle operator to experience unfamiliar brake pedal feeling, and there exists a need for minimizing a bad effect thereby.
For this, differently from conventional brake systems, it is necessary to separate a master cylinder, which reflects the vehicle operator's desire to brake, from wheel cylinders. In other words, a pressure in the master cylinder affected by the vehicle operator's pedal press force should be controlled independently of a pressure in the wheel cylinders.